<apex:page controller="CustopPopupTaxController"  
  title="Search" 
  showHeader="false" 
  sideBar="false" 
  tabStyle="Tax__c" 
  id="pg">

  <apex:form >
  <apex:outputPanel id="page" layout="block" style="margin:5px;padding:10px;padding-top:2px;">
    <apex:tabPanel switchType="client" selectedTab="name1" id="tabbedPanel">

      <!-- SEARCH TAB -->
      <apex:tab label="Search" name="tab1" id="tabOne">

        <apex:actionRegion >  
          <apex:outputPanel id="top" layout="block" style="margin:5px;padding:10px;padding-top:2px;">
            <apex:outputLabel value="Search" style="font-weight:Bold;padding-right:10px;" for="txtSearch"/>
            <apex:inputText id="txtSearch" value="{!searchString}" />
              <span style="padding-left:5px"><apex:commandButton id="btnGo" value="Go" action="{!Search}" rerender="searchResults"></apex:commandButton></span>
          </apex:outputPanel>

          <apex:outputPanel id="pnlSearchResults" style="margin:10px;height:350px;overflow-Y:auto;" layout="block">
            <apex:pageBlock id="searchResults"> 
              <apex:pageBlockTable value="{!results}" var="a" id="tblResults">
                <apex:column >
                  <apex:facet name="header">
                    <apex:outputPanel >Name</apex:outputPanel>
                  </apex:facet>
                   <apex:outputLink value="javascript:top.window.opener.lookupPick2('{!FormTag}','{!TextBox}_lkid','{!TextBox}','{!a.Id}','{!a.Name}', false)" rendered="{!NOT(ISNULL(a.Id))}">{!a.Name}</apex:outputLink>     
                </apex:column>
              </apex:pageBlockTable>
            </apex:pageBlock>
          </apex:outputPanel>
        </apex:actionRegion>

      </apex:tab>

      <!-- NEW Tax TAB -->
      <apex:tab label="New Tax" name="tab2" id="tabTwo">

        <apex:pageBlock id="newTax" title="New Tax" >

          <apex:pageBlockButtons >
            <apex:commandButton action="{!saveTax}" value="Save"/>
          </apex:pageBlockButtons>
          <apex:pageMessages />

          <apex:pageBlockSection columns="2">
            <apex:repeat value="{!$ObjectType.Tax__c.FieldSets.Tax_field_sets_for_Lookup}" var="f">
              <apex:inputField value="{!Tax__c[f]}"/>
            </apex:repeat>
          </apex:pageBlockSection> 
        </apex:pageBlock> 

      </apex:tab>
    </apex:tabPanel>
  </apex:outputPanel>
  </apex:form>
</apex:page>

This is my visualforce page which is showing a unknown property error. I don't know what is the reason. Below I have given my controller. Please Help me out. I have looked some of the discussions here but unable to understand exactly why this is happening.

public with sharing class CustopPopupTaxController {

  public Tax__c t1 {get;set;} // new account to create
  public List<Tax__c> results{get;set;} // search results
  public string searchString{get;set;} // search keyword

  public CustopPopupTaxController() {
    t1 = new Tax__c();
    // get the current search string
    searchString = System.currentPageReference().getParameters().get('lksrch');
    runSearch();  
  }

  // performs the keyword search
  public PageReference search() {
    runSearch();
    return null;
  }

  // prepare the query and issue the search command
  private void runSearch() {
    // TODO prepare query string for complex serarches & prevent injections
    results = performSearch(searchString);               
  } 

  // run the search and return the records found. 
  private List<Tax__c> performSearch(string searchString) {

    String soql = 'select id, name from Tax__c';
    if(searchString != '' && searchString != null)
      soql = soql +  ' where name LIKE \'%' + searchString +'%\'';
    soql = soql + ' limit 500';
    System.debug(soql);
    return database.query(soql); 

  }

  // save the new account record
  public PageReference saveTax() {
    insert t1;
    // reset the account
    t1 = new Tax__c();
    return null;
  }

  // used by the visualforce page to send the link to the right dom element
  public string getFormTag() {
    return System.currentPageReference().getParameters().get('frm');
  }
  
  

  // used by the visualforce page to send the link to the right dom element for the text box
  public string getTextBox() {
    return System.currentPageReference().getParameters().get('txt');
  }

}